The present invention relates to a bidirectional optical transmission method and apparatus therefor using a single optical transmission path.
As a conventional method for bidirectional optical transmission of signals between two stations through an optical transmission path such as an optical fiber and the like, there is a wavelength-division multiplexed (WDM) bidirectional optical transmissions method as described in IEEE, Journal of Lightwave Technology, Vol.7, No. 11, page 1733 to 1740.
An apparatus according to the conventional method of the WDM bidirectional optical transmission is shown in FIG. 1 of the accompanying drawings. In FIG. 1, a base band signal A inputted into an optical transmitting/receiving unit 401 is converted into a light signal of wavelength .lambda..sub.1 at a light source 408, and fed out to an optical transmission path 403 through a wavelength multiplexer/demultiplexer (WMUX/DEMUX) 406 and an optical connector 404. The light signal of wavelength .lambda..sub.1 inputted into an optical transmitting/receiving unit 402 through an optical connector 405 is supplied to a light receiver 411 through an WMUX/DEMUX 407 and is converted into an electrical signal. On the other hand, the base band signal B inputted into the optical transmitting/receiving unit 402 is converted into a light signal of wavelength .lambda..sub.2 at a light source 409, and is fed to the optical transmission path 403 through the WMUX/DEMUX 407 and the optical connector 405. The light signal of wavelength .lambda..sub.2 inputted into the optical transmitting/receiving unit 401 from the optical connector 404 is supplied to a light receiver 410 through the WMUX/DEMUX 406, and is converted into the electrical signal.
When the light signal of wavelength .lambda..sub.1 from the light source 408 passes through the optical connectors 404 and 405, a part of the light signal is reflected. Since the reflected light signal has wavelength of .lambda..sub.1, the signal returns only to the light source 408 by means of the WMUX/DEMUX coupler 406. Accordingly, the light receiver 410 is prevented from receiving interference due to the reflected light. Similarly, a part of the light signal of wavelength .lambda..sub.2 from the light source 409 is reflected when the signal passes through the optical connectors 405 and 404, and the reflected signal returns only to the light source 409 by means of the WMUX/DEMUX coupler 407. Accordingly, the light receiver 411 is also prevented from receiving interference due to the reflected light.
For another bidirectional optical transmission method, a multi-subcarrier bidirectional optical transmission method is described in IEEE, Journal of Lightwave Technology, Vol. 7, No. 11, pages 1819 to 1824. An apparatus according to the conventional method of multi-subcarrier bidirectional optical transmission is shown in FIG. 2. In FIG. 2, a base band signal A inputted into an optical transmitting/receiving unit 501 is introduced to a modulator 517. The modulator 517 frequency-modulates an output signal (a subcarrier signal) of frequency f.sub.l of the subcarrier oscillator 516 in accordance with the base band signal A. The subcarrier signal which is frequency-modulated by the base band signal A is converted into a light signal at a light source 508, and is fed out to an optical transmission path 503 through an optical coupler 506 and an optical connector 504. The light signal inputted into the optical transmitting/receiving unit 502 from an optical connector 505 is supplied on a light receiver 511 through an optical coupler 507, and is converted into an electrical signal. The electrical signal is inputted into a demodulator 519 through a low-pass filter 518 whose cut-off frequency is set to be slightly higher than frequency f.sub.l of the subcarrier oscillator 516, thus the base band signal A is outputted from the demodulator 519.
On the other hand, the base band signal B inputted into the optical transmitting/receiving unit 502 is introduced to a modulator 514. The modulator 514 frequency-modulates an output signal (a subcarrier signal) of frequency f.sub.h of a subcarrier oscillator 515 in accordance with the base band signal B. Now assume that the frequency f.sub.h is higher than f.sub.i. The subcarrier signal frequency-modulated by the base band signal B is converted into a light signal at a light source 509, and is fed out to the optical transmission path 503 through the optical coupler 507 and optical connector 505. The light signal inputted into the optical transmitting/receiving unit 501 from the optical connector 504 is supplied to a light receiver 510 through the optical coupler 506, and is converted into an electrical signal. The electrical signal is inputted into a demodulator 513 through a high-pass filter 512 whose cut-off frequency is set to be slightly lower than frequency f.sub.h of the subcarrier oscillator 515, thus the base band signal B is outputted from the demodulator 513.
When the light signal from the light source 508 passes through the optical connectors 504 and 505, a part of the light signal reflects, and returns to the light receiver 510 through the optical coupler 506. Accordingly, the output of the light receiver 510 is comprised of the frequency-modulated subcarrier signal having a carrier frequency f.sub.h which is intrinsically received and the other frequency-modulated subcarrier signal having a carrier frequency f.sub.l that is an interfering wave due to such reflection. However, the interfering wave is removed by the high-pass filter 512. Similarly, when the light signal from the light source 509 passes through the optical connectors 504 and 505, a part of the light signal reflects and returns to the light receiver 511 through the optical coupler 507. Accordingly, the output of the frequency-modulated light receiver 511 is comprised of the subcarrier signal having a carrier frequency f.sub.l which is intrinsically received and the other having a carrier subcarrier signal having a carrier frequency f.sub.h that is the interfering wave due to such reflection. However, the interfering wave is removed by the low-pass filter 518.
However, in the apparatuses of the conventional methods of bidirectional optical transmission as hereinbefore described, wavelengths of the opposed light sources are set to be different from each other and are free of interference communication due to reflection of light through the use of the WMUX/DEMUX stages, and thus problems exist due to the higher cost of the transmission apparatus necessitated by the expensive WMUX/DEMUX stages and two sets of modulators and demodulators for the subcarrier signals.